function onConnect(socket){

  // 给本次连接的客户端发消息
  socket.emit('hello', 'can you hear me?', 1, 2, 'abc');

  // 给除了本次连接的其他所有连接者发消息
  socket.broadcast.emit('broadcast', 'hello friends!');

  // 给除了本次连接者之外的所有game房间的人发消息
  socket.to('game').emit('nice game', "let's play a game");

  //给除了本次连接者之外的所有game1、game2房间的人发消息
  socket.to('game1').to('game2').emit('nice game', "let's play a game (too)");

  // 给所有game中的人发消息
  io.in('game').emit('big-announcement', 'the game will start soon');

  // 给myNamespace命名空间的所有人发消息
  io.of('myNamespace').emit('bigger-announcement', 'the tournament will start soon');

  //给特定的socketid发消息（私信）
  socket.to(<socketid>).emit('hey', 'I just met you');

  // 发送需要回执的消息
  socket.emit('question', 'do you think so?', function (answer) {});

  // 发送无需压缩的消息
  socket.compress(false).emit('uncompressed', "that's rough");

  // sending a message that might be dropped if the client is not ready to receive messages
  socket.volatile.emit('maybe', 'do you really need it?');

  // sending to all clients on this node (when using multiple nodes)
  io.local.emit('hi', 'my lovely babies');

};

详情 https://blog.csdn.net/nathanhuang1220/article/details/41348213?utm_source=blogxgwz1